1. Field of the Invention
The present invention relates to a method for forming an interconnect, and more particularly to a method for forming an aluminum film used as an interconnect in a semiconductor device.
2. Description of the Related Art
An example of conventional methods for forming an aluminum film used as an interconnect in a semiconductor device is one in which, by means of sputtering or evaporation, atoms or molecules of aluminum struck out from an aluminum target are deposited on a substrate surface. In another example, as reported in 1990 Symposium on VLSI Technology, Digest of Technical Papers, page 5, an aluminum film is deposited on a titanium nitride under a substrate temperature between 230.degree. C. to 350.degree. C. by a low-pressure chemical vapor deposition (LP-CVD) using dimethylaluminum hydride as a source material.
The conventional method for forming interconnects described above suffers from problems in that, in a method for forming an aluminum film for interconnects by sputtering or evaporation, the step-coverage is poor within a contact hole or via-hole which is formed on a substrate surface and which has an aspect ratio larger than 1. This leads to the lowering of reliability of semiconductor devices since the interconnects patterned from the film with the poor step-coverage may involve a non-conduction problem or be fused away due to an increase in the current density.
In a method for forming an aluminum film for interconnects by the low-pressure CVD process, since it is required that the deposition rate be made as high as possible in order to enhance the productivity, the film formation must be performed under a high substrate temperature. Under such condition, when the source molecules of dimethylaluminum hydride reach the substrate surface, most of them decompose and form an aluminum film. Where such a method is applied to the filling of the contact hole or via-hole that has a high aspect ratio and a small diameter, the source molecules reach the bottom of the hole by repeating impingements on sidewalls since the mean free path of the source molecules is longer than the diameter. In this case, since the substrate temperature is high, the source molecules decompose thereby forming a film after the impingement once or twice, so that the practical source material flux at the bottom of the contact hole becomes smaller than that at the upper portion of the contact hole. As the film formation progresses, the diameter of the upper portion of the contact hole or the via-hole becomes smaller due to the film deposited thereat and this makes it more difficult to supply the source material to the bottom portion of the contact holes or the via-holes. This results in the formation of an undesirable void at the bottom portion thereof, in the electrical non-conduction, and in the interconnect fusion due to an increase in the current density thereat, whereby the reliability of semiconductor devices is lowered.